Electro magnetic vibrators of the variable reluctance principle are used i.a., in bone conduction hearing aids and audiometric vibrators for determining hearing thresholds. It is important that such vibrators are: efficient, small, reliable, and are designed in such a way that their properties can be adapted to the particular application. In order to improve conventional bone conduction vibrators with regard hereto the technology has been developed i.a., by means of the invention according to SE 0000810-2.
In spite of improvements in different regards these vibrators suffer from losses, which arise in particular in the iron material conducting the dynamic magnetic flux. These losses may even be larger in the improved constructions described in SE 0000810-2 compared to conventional vibrators of the variable reluctance type.
The losses, which predominantly occur due to eddy currents lead to a deteriorated efficiency and in many cases to an undesired heating up of the iron material. For example, the heat generation can become so high that a short circuit occurs in the windings of the coil. If the vibrator should be used in an implantable bone conduction hearing aid even a small temperature increase could be damaging to the surrounding tissue.
The present invention aims to reducing the problem of eddy currents in the iron material conducting the dynamic magnetic flux in variable reluctance vibrators for bone conduction use.
By means of the new invention a change of the electrical impedance occurs seen in the electrical terminals of the coil making it possible to optimize the function of the vibrator for different applications.